Anode structure for microwave frequency oscillators



May 13, 1969 ANODE STRUCTURE FOR MICROWAVE FREQUENCY OSCILLATORS /0 Q2 4 ave E. T. DOWNING 3,444,429

Filed June 29, 1966 Fla 5 B) ATTORNEX United States Patent 3,444,429 ANODE STRUCTURE FOR MICROWAVE FREQUENCY OSCILLATORS Edward T. Downing, Winchester, Mass., assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed June 29, 1966, Ser. No. 561,480 Int. Cl. H01j 25/58 US. Cl. SIS-39.75 12 Claims ABSTRACT OF THE DISCLOSURE An anode vane member defining a step portion 0t reduced height at the free end adjacent to the cathode member for crossed field oscillator devices.

The present invention relates in general to microwave electron discharge devices and more particularly to such devices of the magnetron type incorporating a multicavity anode structure symmetrically disposed around a cathode and defining therewith an electron interaction system.

In conventional magnetron designs an anode cavity resonator system is defined by a plurality of anode elements in the form of substantially fiat vane wall members joined at their base portions to a common cylindrical wall with the free end portions disposed contiguous to the cathode member. Electrons emitted from the cathode interact with the electric fields established. by the resonator system with a resultant revolving spoke space charge in the interaction region. Such electric fields extend normal to the axis of the cathode member and a magnetic field is required perpendicular to the electron path between the cathode and anode or parallel with the cathode member axis. Hence, such magnetrons are considered to be crossedfield devices.

Magnetic circuit considerations have led in the prior art to the disposition of magnetic pole pieces of a suitably magnetically permeable material such as a soft iron coaxially disposed within the tube envelope adjacent to the anode-cathode interaction region together with externally mounted permanent magnets, conventionally of a C-shaped configuration. The spacing between the ends of the oppositely disposed inner pole piece members defines the air gap in the over-all magnetic circuit. It is considered well known in the art that the weight of the magnetic field producing means is essentially proportional to the square of the field strength (expressed in terms of gauss), to the distance between the magnet poles and to the area of the uniform field. A fuller explanation of the considerations relative to the magnetic circuit in magnetron devices may be found in the text Microwave Magnetrons, Radiation Laboratory Series, McGraw-Hill Book Co., Inc., 1948, pp. 540-557 inclusive.

A crossed-field device of relatively recent origin involves a provision of a second cavity resonator mounted externally to the anode-cathode boundary wall with alternate slots in a common boundary wall for coupling between the two resonator systems. The external cavity resonator becomes the frequency determining element to result in an extremely stable high frequency oscillator under varying load conditions with minimal mode separation problems. conventionally, such electron discharge devices, referred to as the coaxial cavity type magnetron, are provided with an external cavity resonator which operates in a TE dominant mode with the inner resonator system being locked to the external cavity by means of slots to provide the proper conditions for oscillation in the 1r-mode. With the external cavity thus providing the frequency determining means a device at illustratively K band or for example 15-17 kilomegacycles requires an external cavity having 3,444,429 Patented May 13, 1969 the same dimensions for the generation of one kilowatt of high frequency RF power as for kilowatts. Additional information relative to the coaxial type magnetron may be found in United States Letters Patent No. 2,854,603 issued Sept. 30, 1958, to Robert J. Collier et al.

In the generation of high power microwave energy a need has always existed, particularly in beacon radar systems, for a means of reduction of over-all weight of the oscillator. While not limited thereto, the present invention is especially suitable for coaxial cavity type magnetrons for beacon radar applications, as well as man-pack radar systems, in that a large reduction of size and weight of the over-all oscillator device may be realized while maintaining all the prior art frequency stability and power generation capabilities.

It is, therefore, a primary object of the present invention to provide a high frequency oscillator device having a new and improved anode resonator structure which permits a reduction in the size of the magnetic circuit air gap with an accompanying reduction in weight and size of the magnetic field producing means.

Another object of the present invention is the provision of a multicavity inner resonator system in a coaxial cavity type magnetron device defined by a plurality of anode vane elements with the free ends of said elements having a reduced axial height relative to the height thereof at the base ends.

Still another object of the present invention is the provision of a crossed-field device of the magnetron type wherein the inner magnetic circuit members disposed within the tube envelope define an air gap of reduced dimensions comparable to prior art devices yielding similar electrical parameters to result in a minimization of the external field producing requirements.

A feature of the present invention is the provision of an anode structure with normally extending vane members joined at their base portions to a common cylindrical wall and defining a plurality of circumferentially disposed cavity resonators. The free end portions of the vane members are disposed contiguous to an axially disposed cathode member. Inner magnetic pole pieces disposed adjacent opposing ends of the cathode member define an air gap therebetween. By means of a recess or stepped portion defined on opposing sides of each vane member the axial height at the base portion is substantially greater than the height at the free end portions. The inner ends of the magnetic pole piece members are positioned within the wall structure defining the free end recessed portions to result in a magnetic air gap of reduced dimension. As aforementioned, the weight of the magnetic field producing means bears a linear relationship to the length of the magnetic gap. Reduction of the latter therefore has resulted in substantial reductions in the over-all magnetic field requirement while maintaining the requisite strength for the generation of high power high frequency oscillations. A unique result observed in the practice of the invention is that the unloaded Q characteristics with the new anode structure remain substantially the same as the prior art anode structures having a larger magnetic air gap dimenslon.

Other objects, features and advantages will be evident after consideration of the following detailed description together with the accompanying drawings wherein:

FIG. 1 is a fragmentary enlarged cross-sectional view of the prior art anode structure utilized in a coaxial cavity type magnetron;

FIG. 2 is a fragmentary enlarged cross-sectional view illustrative of the anode structure of the present invention;

FIG. 3 is a partial cross-sectional view of a magnetron device illustrative of an embodiment of the present invention;

FIG. 4 is an enlarged fragmentary perspective view of a portion of the anode-inner cavity resonator system shown in FIG. 3 in the area indicated generally by the arrow and reference letter A; and

FIG. 5 is a fragmentary cross-sectional view of an alternative embodiment of the present invention.

Referring now to the drawings, FIG. 1 is an exemplary embodiment of the prior art anode structure together with the adjacent magnetic circuit components. A plurality of anode vane elements 2 is supported at their base end by common cylindrical wall member 4 and extends normal to the axis of the tube. Outer coaxial cavity resonator 6 is coupled to the inner cavity resonator system defined by the anode structure by means of coupling slots 8 circumferentially disposed in wall member 4 and communicating with alternate anode cavity resonators. A dampingelement of a high resistance material 10 is desirably provided in such devices to suppress the generation of undesirable oscillation modes in the operation of the over-all tube. The inner magnetic circuit elements comprise two coaxially mounted pole piece members 12 and 14 which are axially aligned in a spaced apart rela tionship to thereby form a magnetic air gap indicated by the symbol l in this illustration. The gap provides for a region of strong uniform magnetic field strength in the region of interaction between the anode and cathode members which is required to produce the high frequency oscillations.

It will be noted that anode vane elements 2 are substantially fiat and rectangular in accordance with the teachings of the aforementioned United States Patent No. 2,854,603. These elements are approximately an electrical quarter wavelength long in the range of the frequencies of the current flowing along the anode cylindrical wall 4 which forms a common boundary between the outer external cavity resonator and the inner resonator systems of such devices. In accordance with the well known principles of the operation of coaxial cavity type magnetrons electric currents, particularly in the TE mode, flow circumferentially along the inner common boundary wall 4 of the external cavity resonator 6 and the slots 8 which communicate with alternate anode cavity resonators provide a means for the generation of high voltages at the tips of the vane elements adjacent to the cathode member in an alternating manner to result in the 1r mode of oscillation.

Attention is directed to the axial height of the prior art anode vane element configuration which results in the tips of the inner magnetic pole piece members 12 and 14 being disposed above a reference plane taken along the parallel horizontal peripheral walls 16 and 18 defined by the vane elements. Since, as aforesaid, the length of the magnetic air gap is a controlling factor in the determination of the magnetic circuit parameters to produce the required magnetic field strength in such devices, it is to this parameter that consideration is focused in the present invention to advantageously provide a means for reduction of the weight of the over-all magnetic circuit components.

Referring next to FIG. 2 of the drawings, the anode vane element of the present invention is designated generally by numeral 20. The external cavity resonator, coupling slots and mode suppressor, will be similar to the structure shown in FIG. 1 and have been similarly designated. Further, the anode vane element 20 is secured at its base end to a common anode cylindrical wall member 4. In accordance with the teachings of the present invention anode vane elements 20 are provided with parallel horizontal upper and lower peripheral walls 22 and 24 extending normally towards the free end portion 26. A recessed or stepped portion 25 is defined in opposing walls 22 and 24 by means of a right angular notch provided by walls 28 and 30. Calculations of the electrical length of the resultant anode 'vane element structure, as well as operation in illustrative devices of the present invention, have demonstrated that the electric field configurations necessary for the operation in the desired mode are maintained and are in fact more efliciently confined within the cathode-anode interaction region adjacent to the free ends 26. The axial height of the anode vane elements taken at the base end 32 may, for example, in a K, band coaxial cavity magnetron, have a value of .130 inch, while the axial height at the free end is substantially reduced to a value of .070 inch. The measured unloaded Q values in devices incorporating such an anode structure will be in the range of 3000 or higher which is required for the operation of such oscillator devices.

The inner magnetic pole piece members 34 and 36 have their inner end tip portions 38 and 40 positioned closer together and below the reference plane defined by the upper and lower horizontal walls 22 and 24 of the vane element. The resultant magnetic air gap l has accordingly been substantially reduced with a resultant reduction in the magnetic circuit element requirements. In an exemplary device embodying the teachings of the present invention external magnets mounted on the tube envelope have been reduced in weight from a value of approximately 10 lbs. for prior art devices of comparable frequency and power level to a value of 5 lbs. or less. The magnetic field strength produced was in the range of 5000 to 6000 gauss.

An exemplary microwave oscillator 42 of the coaxial cavity magnetron type under consideration embodying the structure of the present invention is illustrated in FIGS. 3 and 4. The coaxial cavity type magnetron 42 comprises an envelope 44 defining upper and lower annular cover members 46 and 48 which are hermetically sealed in order that the over-all tube assembly may be evacuated. A cylindrical electron emissive cathode member 50 is centrally disposed within the inner cavity resonator system including a plurality of anode vane elements 20 fabricated in accordance with the present invention joined at their base ends to cylindrical wall member 4. The anode vane elements 20 extend normally to the cathode member axis and define therebetween a plurality of cavity resonators 52.

The cathode 50 is supported by a cylindrical member 54 secured in the conventional manner to annular member 56 and insulatedly secured by means of dielectric member 58 to metallic annular member 60 and magnetic pole piece member 36. Outer terminal member 62 provides for the energizing of a heater coil disposed within the cathode member 50 by suitable high voltages.

Inner magnetic pole piece member 34 is axially disposed opposite to pole piece member 36 and external C-shaped permanent magnet members 64 contact pole piece adapters 66. The magnets 64 are disposed at substantially right angles to the surface of the drawing as viewed by the reader. The disposition therefore shown is rotated from the actual mounting position and is illustrated in this manner for the purposes of the description to include all applicable structure necessary for the operation of a magnetron oscillator. The magnetic circuit extends parallel to the axis of the cathode member and provides the required magnetic field strengths within the anode-cathode interaction region indicated generally by the numeral 68 as well as the external cavity resonator 6. The electric field currents are disposed in a direction perpendicular to the magnetic fields to evolve a crossed-field configuration.

The remainder of the illustrative embodiment comprises cooling fins 70 and exhaust tubulation 72 for evacuation of the over-all device. In addition, output waveguide 74, including a conventional transformer section and mounting plate 76 for supporting the tube in a socket arrangement is provided.

The dimensions of external cavity resonator 6 are selected to provide high frequency oscillations in a predetermined frequency range with the TE mode of operation preferred in such devices. The microwave RF energy generated is coupled to an external load by means of the waveguide 74.

For purposes of clarification the components of the inner cavity resonator system including the anode vane elements 20 joined to anode cylindrical wall 4 and slots 8 communicating with the external cavity resonator system are illustrated in exploded form in FIG. 4. The general area illustrated is designated by the symbol A in FIG. 3.

In FIG. 5 an alternative embodiment of the anode vane element of the present invention is illustrated and designated generally 80. In this embodiment the axial height at the free end 82 is again substantially reduced relative to the base end 84 joined to the common anode cylindrical wall member 4. The inner magnetic pole piece members 34 and 36, it will be noted, have gradually tapering inner tip portions 86 and 88. The recessed or stepped notches 90 are defined on opposing sides of the vane element 80 in substantially the same manner as that shown in FIG. 2. The longer extending horizontal walls 92 and 94 and the wall portions 96 and 98 are therefore illustrated. In this modification peripheral wall portions 92 and 96 as well as 94 and 98 are joined by tapered sections 100 and 102 of a gradually reducing dimension to define an obtuse angular configuration. The angle of the taper may follow closely that of the tapered sides 86 and 88 of the pole pieces 34 and 36 respectively. It is believed that increased efficiency of interaction between the electrons in the interaction space and the crossed electric and magetic fields will follow the utilization of this tapered configuration.

This completes the description of specific embodiments of the present invention together with an illustrative magnetron oscillator utilizing the present structure and it will be evident to those skilled in the art that numerous modifications or alterations may be practiced. The foregoing description and appended drawings are intended as exemplary teachings without in any manner limiting the scope and breadth of the invention as defined in the accompanying claims.

What is claimed is:

1. A microwave frequency oscillator comprising:

an electron emissive cathode member;

an anode wall member having a plurality of normally extending vane elements joined at one end thereto defining therebetween a plurality of circumferentially disposed cavity resonators adjacent to but spaced from said cathode member;

said anode vent elements having wall structure in parallel horizontal peripheral walls defining a stepped portion contiguous to said cathode member;

means disposed on opposing sides of said cavity resonators for establishing a magnetic field in a direction parallel to the axis of said cathode member;

said magnetic field producing means including spaced apart pole piece members defining therebetween a magnetic air gap, the length of said air gap being less than the axial height of said anode vane elements at the point where they are joined to said anode wall member.

2. A microwave frequency oscillator comprising:

an electron emissive cathode member;

an anode member including a cylindrical wall member and a plurality of normally extending vane elements having their base ends connected to said wall member and free ends remote therefrom to define therebetween a plurality of circumferentially disposed cavity resonators adjacent to but spaced from said cathode member;

said vane elements having wall structure in parallel horizontal peripheral walls contiguous to the free ends defining a stepped portion to thereby abruptly reduce the axial height of said vane elements at said free ends relative to the base ends.

3. A microwave frequency oscillator in accordance with claim 2 wherein said stepped portion includes vertical and horizontal walls defining a right angular configuration.

4. A microwave frequency oscillator in accordance with claim 2 wherein said stepped portion includes a parallel horizontal wall joined to the horizontal peripheral walls contiguous to the base end of said vane elements by a gradually tapering section.

5. A coaxial cavity magnetron comprising:

an electron emissive cathode member;

an anode member having a plurality of normally extending vane elements defining therebetween an inner cavity resonator system disposed adjacent to said cathode member and defining therewith an anodecathode electron interaction space;

an annular external cavity resonator disposed coaxially to said anode member;

means for coupling microwave energy between said inner and external cavity resonators;

means disposed on opposing side of said interaction space for establishing a magnetic field in a direction perpendicular to the electron path in said interaction space;

said magnetic field producing means including spaced apart pole piece members defining therebetween a magnetic air gap;

the opposing ends of said pole piece members being positioned below the plane of the uppermost peripheral walls of said vane elements taken in a direction normal to the axis of said cathode member.

6. A coaxial cavity magnetron in accordance with claim 5 wherein said anode vane elements define wall structure in parallel horizontal peripheral walls having a right angular stepped portion contiguous to said cathode member.

7. A coaxial cavity magnetron in accordance with claim 5 wherein said anode vane elements define Wall structure in parallel horizontal peripheral walls providing a stepped portion having an obtuse angular configuration contiguous to said cathode member.

8. A coaxial cavity magnetron in accordance with claim 5 wherein said anode member comprises a cylindrical wall common to the inner and external cavity resonators and each of said vane elements have a base end connected to said wall and a free end contiguous to said cathode member, the height of said base end taken in a direction parallel to the axis of said cathode member being substantially greater than the height at the free end.

9. A coaxial cavity magnetron in accordance with claim 5 wherein each of said anode vane elements define a stepped portion in opposing sides adjacent to said electron interaction space.

10. A crossed field electron discharge device comprismg:

an evacuated envelope;

a vane type anode cavity resonator and cathode interaction region within said envelope;

means for producing transverse electric and magnetic fields in said interaction region;

an external cavity resonator disposed coaxially with respect to said interaction region;

means including magnetic pole piece members disposed inside said envelope adjacent to opposing ends of said anode cavity resonator and defining therebetween an air gap the dimensions of which are less than the height of said anode vanes at a point adjacent to said external cavity resonator.

11. A crossed field electron discharge device in accordance with claim 10 wherein each anode vane is provided in the end adjacent to the cathode with right angular notches within which the opposing ends of said pole piece members are disposed.

12. A crossed field electron discharge device in accordance with claim 10 wherein each anode vane is provided in the end adjacent to the cathode with recessed portions of an obtuse angular configuration with the opposing ends of said pole piece members positioned within said recesses.

References Cited UNITED STATES PATENTS 5 6/1959 Revel-din 31539.75 X 12/1960 Yu et a1. 31539.65 2/ 1965 Drexler et a1 315-39.77 7/1966 Peasley et al. 315-3977 X 10 8 FOREIGN PATENTS 1,250,502 12/1960 France.

HERMAN KARL SAALBACH, Primary Examiner.

SAXFIELD CHATMON, JR., Assistant Examiner.

US. Cl. X.R. 

